Method of controlling the corrosion rate of alloy particles, alloy particle with controlled corrosion rate, and articles comprising the particle

ABSTRACT

A composite particle comprises a core, a shielding layer deposited on the core, and further comprising an interlayer region formed at an interface of the shielding layer and the core, the interlayer region having a reactivity less than that of the core, and the shielding layer having a reactivity less than that of the interlayer region, a metallic layer not identical to the shielding layer and deposited on the shielding layer, the metallic layer having a reactivity less than that of the core, and optionally, an adhesion metal layer deposited on the metallic layer.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. Non-provisional applicationSer. No. 13/194,271 filed on Jul. 29, 2011. The parent application isincorporated by reference herein in its entirety.

BACKGROUND

Certain downhole operations involve placement of elements in a downholeenvironment, where the element performs its function, and is thenremoved. For example, elements such as ball/ball seat assemblies andfracture (frac) plugs are downhole elements used to seal off lower zonesin a borehole in order to carry out a hydraulic fracturing process (alsoreferred to in the art as “fracking”) to break up different zones ofreservoir rock. After the fracking operation, the ball/ball seat orplugs are then removed to allow fluid flow to or from the fracturedrock.

Balls and/or ball seats, and frac plugs, may be formed of a corrodiblematerial so that they need not be physically removed intact from thedownhole environment. In this way, when the operation involving theball/ball seat or frac plug is completed, the ball, ball seat, and/orfrac plug corrodes away. Otherwise, the downhole article may have toremain in the hole for a longer period than is necessary for theoperation.

To facilitate removal, such elements may be formed of a material thatreacts with the ambient downhole environment so that they need not bephysically removed by, for example, a mechanical operation, but mayinstead corrode or dissolve under downhole conditions. However, whilecorrosion rates of, for example, an alloy used to prepare a corrodiblearticle can be controlled by adjusting alloy composition, an alternativeway of controlling the corrosion rate of a downhole article isdesirable.

Corrodible materials may include those having a high activity on thesaltwater galvanic series, such as a magnesium alloy adjusted forcorrosion rate. It has been found that adjusting the amount of tracecontaminants in a magnesium alloy can have a significant impact on thecorrosion rate of such alloys (Song, G. and Atrens, A., “UnderstandingMagnesium Corrosion: A Framework for Improved Alloy Performance,” Adv.Eng. Mater. 2003, 5(12) pp. 837-858). For example, metals such asnickel, iron, copper, calcium, etc., may be added to magnesium toincrease the corrosion rate and other metals such as zirconium, yttrium,etc. may be added to decrease the corrosion rate. Balancing the amountsof such additives to achieve a desired bulk corrosion rate can in thisway control overall corrosion of articles made from the alloy; however,such an approach requires preparation of multiple batches of alloy,requiring high batch-to-batch reproducibility and precise, reproduciblecontrol of metal additives or contaminants in the alloy.

There accordingly remains a need for controlling the overall corrosionrate of magnesium alloys for use in downhole articles without need forfine adjustment of alloy composition and with improved corrosioncontrol.

SUMMARY

The above and other deficiencies of the prior art are overcome by, in anembodiment, a composite particle comprising a core, a shielding layerdeposited on the core, and further comprising an interlayer regionformed at an interface of the shielding layer and the core, theinterlayer region having a reactivity less than that of the core, andthe shielding layer having a reactivity less than that of the interlayerregion, a metallic layer not identical to the shielding layer anddeposited on the shielding layer, the metallic layer having a reactivityless than that of the core, and optionally, an adhesion metal layerdeposited on the metallic layer.

In another embodiment, a composite particle comprises amagnesium-aluminum alloy core, a shielding layer comprising analuminum-containing layer deposited on the core, and further comprisingan interlayer region comprising α-Mg and γ-Mg₁₇Al₁₂ formed at theinterface between the magnesium alloy core and the aluminum-containinglayer, and further comprising inclusions of alumina, magnesia, or acombination comprising at least one of these oxides, a metallic layerdeposited on the shielding layer, the metallic layer comprising Ni, Fe,Cu, Co, W, alloys thereof, or a combination comprising at least one ofthe foregoing, an aluminum-containing shielding layer deposited on themetallic layer, and optionally, an aluminum-containing adhesion metallayer, wherein the interlayer region, shielding layer, metallic layer,and optional adhesion metal layer are inter-dispersed with each other.

In another embodiment, a method of adjusting corrosion rate in anaqueous electrolyte is disclosed for a composite particle having a core,a shielding layer deposited on the core, and further comprising aninterlayer region formed at an interface of the shielding layer and thecore, the interlayer region having a reactivity less than that of thecore, and the shielding layer having a reactivity less than that of theinterlayer region, a metallic layer not identical to the shielding layerand deposited on the shielding layer, the metallic layer having areactivity less than that of the core, and optionally, an adhesion metallayer deposited on the metallic layer, the method comprising selectingthe metallic layer such that the lower the reactivity of the metalliclayer is relative to the shielding layer, the greater the corrosionrate, and selecting the amount, thickness, or both amounts andthicknesses of the shielding layer and metallic layer such that the lessthe amount, thickness, or both amount and thickness of the shieldinglayer relative to those of the metallic layer, the greater the corrosionrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings wherein like elements are numbered alikein the several Figures:

FIG. 1 shows a cross-sectional view of a composite particle 100 a havinga multilayer structure (FIG. 1A) and a cross-sectional view of acomposite particle 100 b having an inter-dispersed layer (FIG. 1B);

FIG. 2 shows a cross-sectional view of a composite particle 200 a havinga multilayer structure (FIG. 2A) and a cross-sectional view of acomposite particle 200 b having an inter-dispersed layer (FIG. 2B); and

FIG. 3 shows a cross-sectional view of an exemplary corrodible downholearticle 300 prepared from the composite particles 310.

DETAILED DESCRIPTION OF THE INVENTION

Disclosed herein is a composite particle useful for fabricating acorrodible article. The composite particle has multilayered structure ofa core of a high reactivity material, such as magnesium or a magnesiumalloy, coated a shielding layer such as for example, aluminum. At theinterface of the shielding layer and the core, an intermetallic phasecan form, such as a crystalline metallic compound of magnesium andaluminum, and present in discontinuous regions. The shielding layer,which includes the intermetallic regions, has a layer of a noblematerial with a lower reactivity (i.e., more noble than the shieldinglayer, though comparable in reactivity to the intermetallic phase)disposed on it. An additional layer of an adhesive metal, for example,aluminum, can be disposed over the noble material layer, to provideadhesion between particles upon molding. The interlayer region,shielding layer, noble material layer (referred to herein as the“metallic layer” where the noble material is a metal), and optionaladhesion layer are believed to be inter-dispersed with each other, andform a compositionally varied outer shell which is also inter-dispersedwith the core.

The noble material layer, which has a lower reactivity relative to thecore material, acts as a cathode, whereas the core, made of a metal suchas magnesium which is more reactive than the noble metal layer, isanodic relative to the noble metal layer. The shielding layer, whichincludes the intermetallic phase, is also cathodic relative to the core,but anodic relative to the noble metal layer. A galvanic discharge cycle(e.g., corrosion) occurs between the relatively anodic and relativelycathodic materials in the presence of an electrolyte. By adjusting thecomposition of the noble metal layer relative to the core and shieldinglayers, and by adjusting the amounts and/or thicknesses of the shieldingand noble metal layers, the corrosion rate of the composite particle isadjusted.

The composite particles are formed into articles by compressing andshaping the particles using, for example, cold molding followed byforging.

The core includes any material suitable for use in a downholeenvironment, provided the core is corrodible in the downhole environmentrelative to a second material having a different reactivity. In anembodiment, the composite particle thus includes a magnesium-containingcore. A magnesium-containing core includes any such alloy which iscorrodible in a corrosive environment including those typicallyencountered downhole, such as an aqueous environment which includes salt(i.e., brine), or an acidic or corrosive agent such as hydrogen sulfide,hydrochloric acid, or other such corrosive agents. Magnesium alloyssuitable for use include alloys of magnesium (Mg) with aluminum (Al),cadmium (Cd), calcium (Ca), cobalt (Co), copper (Cu), iron (Fe),manganese (Mn), nickel (Ni), silicon (Si), silver (Ag), strontium (Sr),thorium (Th), zinc (Zn), zirconium (Zr), or a combination comprising atleast one of these elements. Particularly useful alloys includemagnesium alloy particles including those prepared from magnesiumalloyed with Ni, W, Co, Cu, Fe, or other metals. Alloying or traceelements can be included in varying amounts to adjust the corrosion rateof the magnesium. For example, four of these elements (cadmium, calcium,silver, and zinc) have mild-to-moderate accelerating effects oncorrosion rates, whereas four others (copper, cobalt, iron, and nickel)have a still greater accelerating effect on corrosion. Exemplarycommercially available magnesium alloys and which include differentcombinations of the above alloying elements to achieve different degreesof corrosion resistance include but are not limited to, for example,magnesium alloyed with aluminum, strontium, and manganese such as AJ62,AJ50x, AJ51x, and AJ52x alloys, and magnesium alloyed with aluminum,zinc, and manganese which include AZ91A-E alloys.

It will be appreciated that alloys having corrosion rates greater thanthose of the above exemplary alloys are contemplated as being usefulherein. For example, nickel has been found to be useful in decreasingthe corrosion resistance (i.e., increases the corrosion rate) ofmagnesium alloys when included in amounts of less than or equal to about0.5 wt %, specifically less than or equal to about 0.4 wt %, and morespecifically less than or equal to about 0.3 wt %, to provide a usefulcorrosion rate for the corrodible downhole article. In anotherembodiment, the magnesium-containing core comprises a magnesium-aluminumalloy.

Particle sizes for the magnesium alloy cores may be from about 50 toabout 150 micrometers (μm), more specifically about 60 to about 140 μm,and still more specifically about 70 to about 130 μm. Useful magnesiumalloys may include combinations of the above elements and/orcontaminants sufficient to achieve a corrosion rate for the magnesiumalloy core of about 0.1 to about 20 mg/cm²/hour, specifically about 1 toabout 15 mg/cm²/hour using aqueous 3 wt % KCl solution at 200° F. (93°C.).

The composite particle includes a shielding layer. The shielding layeris formed by depositing on the core, a material having a lowerreactivity than that of the core. In an exemplary embodiment, theshielding layer is an aluminum-containing layer deposited on the core.In an embodiment, the core is a magnesium alloy core and the shieldinglayer is an aluminum-containing layer. As used herein “on” and“deposited on” mean that a layer may or may not be in direct contactwith, the underlying surface to which the layer is applied, unlessotherwise specified as by stating that the layers are at least partiallyin contact. It will be further understood that “deposited” and“depositing,” when used in with respect to a method, indicates theaction of deposition, whereas “deposited” when used in the context of acomposition or article, merely indicates the juxtaposition of the layerwith respect to the substrate and does not indicate a process ofdeposition. The shielding layer further comprises an interlayer regionformed at the interface of the core and shielding layer, which iscompositionally derived from the core and shielding layers. In anembodiment, the interlayer region forms at the boundary of amagnesium-containing core and an aluminum-containing shielding layer,and the interlayer region comprises an intermetallic compound. Forexample, magnesium-aluminum alloys include an α-Mg phase, and inaddition, a γ-Mg₁₇Al₁₂ intermetallic phase which accumulates at thegrain boundaries within the Mg—Al alloy. The intermetallic γ-Mg₁₇Al₁₂phase is generally present in amounts of less than 30 wt % of the Mg—Alalloy. Depending upon the composition, additional phases can also bepresent, including solid solution Al, and other intermetallic phasessuch as β-Mg₂Al₃. Upon deposition of the aluminum-containing shieldinglayer, the γ-Mg₁₇Al₁₂ phase forms and accumulates as well at theinterface of the shielding layer and the Mg-containing core. Thermaltreatment can accelerate the formation of the interlayer region. Forexample, heating at temperatures less than the eutectic point (e.g.,less than or equal to about 450° C., depending on the alloy composition,and as long as the eutectic point is not exceeded) for about 15 minutescan form an intermetallic phase at the interface of the Mg-containingcore and the Al-containing layer. The composite particle thus includes,as part of the interlayer region, the intermetallic compound γ-Mg₁₇Al₁₂.The interlayer region forms over the entire contacting area of theMg-containing core and the Al-containing layer, or a portion of thecontacting areas. Deposition method and any heat treating can beadjusted so that the intermetallic phase intervenes between a portion ofcontacting surfaces of the Mg alloy core and the Al-containing layer.The shielding layer further includes an oxide of one or more of themetals of which the core and/or shielding layers are comprised. Forexample, where the core comprises magnesium or a magnesium-aluminumalloy, and the shielding layer comprises aluminum, the shielding layeroptionally includes oxides of magnesium (such as magnesia), aluminum(such as alumina), or a combination comprising at least one of theforegoing.

The composite particle further includes a metallic layer not identicalto the shielding layer and deposited on the shielding layer. Themetallic layer has a lower reactivity relative to the core, based on thesaltwater galvanic series from lower reactivity (i.e., more noblemetals) to high reactivity (i.e., less noble metals). In an embodiment,the metal(s) used for the metallic layer allow for the formation ofhydrogen when used as a cathode in an electrochemical cell. The metalliclayer thus comprises a group 6-11 transition metal. Specifically, thegroup 6-11 transition metal includes Ni, Fe, Cu, Co, W, alloys thereof,or a combination comprising at least one of the foregoing.

The composite particle optionally includes an adhesion layer depositedon the metallic layer. The adhesion layer comprises a material whichpromotes adhesion between the composite particles. An exemplary adhesionlayer includes aluminum or an aluminum alloy. Upon compressing andforging of the adhesion layer-coated composite particles to form amolded article, the particles bind to one another through interparticlecontact via the material of the adhesion layer, to further providemechanical strength to the article.

The layers (shielding layer, metallic layer, and optional adhesionlayer) may each have an average thickness of about 0.05 to about 0.15μm, and specifically about 0.07 to about 0.13 μm. In an embodiment, eachlayer does not completely cover the underlying layer, and the layercoverage is thus discontinuous. Furthermore, where the layers are “on”one another, interstitial spaces at the interfaces of the layers may bepresent. In an embodiment, the interlayer region, shielding layer,metallic layer, and optional adhesion metal layer are inter-dispersedwith each other. As used herein, “inter-dispersed” mean that two or moreadjacent layers interpenetrate into or through each other in intimateadmixture, where it will be appreciated that two (or more)inter-dispersed layers have, on average, a compositional gradient due tothe interpenetration of one layer into the adjacent layer.

The core and shielding layer, shielding layer and metallic layer, andmetallic layer and optional adhesion layer, are each thus in mutualpartial contact, and are inter-dispersed, such that components of thecore, the shielding layer, and the metallic layer are present at theexposed surface of the composite particle.

In an embodiment, the composite particles have a corrosion rate of about0.1 to about 20 mg/cm²/hour, specifically about 1 to about 15mg/cm²/hour using an aqueous 3 wt % KCl solution at 200° F. (93° C.).

In a specific embodiment, the shielding layer is an aluminum-containinglayer, and the core is a magnesium-containing core. In an embodiment,the shielding layer comprises aluminum. In another embodiment, theshielding layer further comprises, in addition to aluminum, inclusionsof alumina, magnesia, or a combination comprising at least one of theseoxides. The shielding layer further includes an interlayer region formedat the interface between the magnesium alloy core and the firstaluminum-containing layer. In an embodiment, the interlayer regioncomprises γ-Mg₁₇Al₁₂.

Also in a specific embodiment, the metallic layer comprises a group 6-11transition metal on the shielding layer. The group 6-11 transition metalincludes Ni, Fe, Cu, Co, W, alloys thereof, or a combination comprisingat least one of the foregoing.

Optionally, in a specific embodiment, the adhesion layer is analuminum-containing layer deposited on the metallic layer.

Deposition of the shielding, metallic, and adhesion layers on the coreis not particularly limited. Where either or both of the shielding andoptional adhesion layers includes aluminum or an aluminum alloy,uniformly depositing the aluminum layer(s) on a magnesium alloy coreparticle is accomplished in one exemplary embodiment by decomposition ofan organometallic compound, such as triethylaluminum (having a boilingpoint of 128-130° C. at 50 mm Hg), after introducing the organometalliccompound into a fluidized bed reactor containing the magnesium alloycore particles to deposit the shielding layer, or the introducing theorganometallic compound into a reactor containing magnesium alloycore/shielding layer/metallic layer particles to deposit the adhesionlayer. The interlayer region, which in this case includes anintermetallic compound such as γ-Mg₁₇Al₁₂, further forms at theinterface of the Mg alloy core and the shielding layer by a thermaltreatment, such as sintering and/or annealing, and/or forging of anarticle molded from the composite particles, at a temperature below themelting point of all or part of the composite particle.

Similarly, depositing a uniform metallic layer is also not particularlylimited, and may be accomplished by, for example, decomposition of anorganometallic compound (such as nickel carbonyl where the metalliclayer is nickel) after introducing the organometallic compound into afluidized bed reactor containing the magnesium alloy core particlescoated with the shielding layer.

The core can also be coated with materials for the shielding layer,metallic layer, and optional adhesion layer using a physical mixingmethod. For example, the core can be admixed with one or more componentsof the shielding layer, metallic layer, and optional adhesion layer bycryo-milling, ball milling, or the like. In this way, the shielding,metallic layer and adhesion layer components can be includedsequentially, or components for two or all three layers includedsimultaneously. Combinations of deposition methods including vapor phasedeposition and physical methods can also be used to provide thecomposite particles. Where all components are included by physicalmixing simultaneously, it will be appreciated that a single layer isformed which is a composite of the shielding layer, metallic layer, andadhesion layer components.

In another embodiment, the core comprises an inner core of a first corematerial and an outer core of a second core material, the inner corematerial having a lower reactivity than that of the outer core. Theinner core is any material useful for depositing thereon a highreactivity material such as magnesium, without limitation. The innercore can thus be any suitable, low reactivity material, such as a 6-11transition metal including Ni, Fe, Cu, Co, W, alloys thereof, or acombination comprising at least one of the foregoing; a metal oxide suchas alumina, silica, silicates, iron oxides, titania, tungstates, and thelike; a polymer including a phenolic polymer; ceramics; glasses; orother such materials. In an exemplary embodiment, the inner corecomprises an aluminum alloy, nickel, iron, alumina, titania or silica,and the outer core comprises magnesium or a magnesium alloy as describedhereinabove. The outer core is deposited on the inner core using anysuitable deposition method such as physical vapor deposition (PVD) ofthe metallic magnesium or magnesium alloy in a fluidized bed reactor.The core structure having inner and outer cores is then coated withshielding layer, metallic layer, and optional adhesion layer asdescribed above to form the composite particle.

The composite particle generally has a particle size of from about 50 toabout 150 micrometers (μm), and more specifically about 60 to about 140μm.

In another embodiment, a method of adjusting corrosion rate of acomposite particle, or article prepared therefrom, is disclosed. In anembodiment, adjusting is accomplished by either or both of selecting thecomposition of the metallic layer to have the desired reactivity, wherethe lower the reactivity of the metallic layer relative to the shieldinglayer (and by definition, to both the core and interlayer region), thefaster the corrosion rate; and conversely, the higher the reactivity ofthe metallic layer relative to the shielding layer, the slower thecorrosion rate. Alternatively or in addition, in an embodiment,adjusting is accomplished by increasing the amount and/or thickness ofthe shielding layer for any given amount and/or thickness of metalliclayer. It will further be appreciated that additional control of thecorrosion rate is accomplished by the degree of inter-dispersion of thecore, interlayer region, shielding layer, and metallic layer, where themore highly inter-dispersed these layers are, the greater the corrosionrate, and conversely, the less inter-dispersed the layers, the slowerthe corrosion rate. Thus, amount and thickness as used herein arerelated in that the higher the amount of a layer, expressed as weightpercent based on the weight of the composite particle, the greater thethickness.

The surface of the composite particles includes both anodic and cathodicregions of the inter-dispersed layers. It will be understood that“anodic regions” and “cathodic regions” are relative terms, based on therelative reactivity of the inter-dispersed materials. For example asdiscussed above, magnesium (from the core) is anodic relative to thecathodic intermetallic compound of the interlayer region (γ-Mg₁₇Al₁₂)and cathodic aluminum from the interlayer region/shielding layer, andanodic relative to nickel from the cathodic metallic layer. Similarly,intermetallic compound (γ-Mg₁₇Al₁₂) is anodic relative to cathodicaluminum from the shielding layer, and anodic relative to nickel fromthe cathodic metallic layer; and aluminum from the shielding layer isanodic relative to nickel from the metallic layer.

In this way, upon exposure of the surface of the composite particle (andany article made from the composite particles) to an electrolyte,multiple localized corrosion mechanisms take place in which reversal ofanodic and cathodic regions occur. For example, after exposed anodiccore material (such as magnesium) is corroded, a previously cathodicmaterial (such as intermetallic compound or aluminum in the shieldinglayer) becomes anodic and is corroded by interaction with the morecathodic metallic layer (e.g., which includes nickel, etc.). As thesurface corrodes away and new, more anodic core material such asmagnesium is exposed, the situation again reverses and the aluminum orintermetallic compound becomes cathodic toward the core material.

As corrosion advances in localized regions on the surface between anodicand cathodic regions in the presence of an electrolyte fluid (water,brine, etc.), these regions, referred to herein as micro-cells, cancorrode outward over the surface of the composite particle and link toother micro-cells to form larger corrosion regions, which in turn canlink to other corrosion regions, etc., as further anodic materials suchas magnesium (from the core) or intermetallic γ-Mg₁₇Al₁₂ (from theinterlayer region/shielding layer) is exposed. After these regionscorrode, new, underlying anodic materials from the core are exposed tothe electrolyte. Upon corroding, these inter-dispersed layers can thusbecome permeable to the electrolyte fluid. This allows percolation ofelectrolytic fluids into the corroding surface to penetrate andundermine the layers, and the process repeats until the corrodiblematerials are consumed. It will be appreciated that the presence ofmetal oxides at the core/shielding layer interface also decreases thecorrosion rate of the core at the interface by acting as an inertbarrier, and thus affects the relative anodic/cathodic character of themicro-cell (for example, where alumina and/or magnesia are presentbetween a magnesium core and the intermetallic compound, theintermetallic compound is insulated from the core and will be anodicrelative to the metallic layer). In this way, the presence of inclusionsof metal oxides affects the overall corrosion rate of the compositeparticle.

Where the core comprises an inner and outer core in which the outer coreis anodic, corrosion advances until only the inner core remains. Theinner core thus exposed no longer has the structural integrity andcohesiveness of the composite particle, and disperses into thesurrounding fluid as a suspension of particles, and can be removed inthis way.

Thus, in an embodiment, a method of adjusting corrosion rate in acomposite particle includes selecting the metallic layer such that thelower the reactivity of the metallic layer is relative to the shieldinglayer, the greater the corrosion rate. In another embodiment, a methodof adjusting corrosion rate in a composite particle includes selectingthe amount, thickness, or both amounts and thicknesses of the shieldinglayer and the metallic layer such that the less the amount, thickness,or both amount and thickness of the shielding layer are relative tothose of the metallic layer, the greater the corrosion rate. Theinterlayer region, shielding layer, metallic layer, and optionaladhesion metal layer being inter-dispersed with each other, and havecompositions as discussed above.

In another embodiment, an article comprises the composite particleswhich may be provided as a powder or other suitable form such as apre-compressed pellet. Articles may be prepared from the compositeparticle by compressing or otherwise shaping the composite particles, toform an article having the appropriate shape. For example, the compositeparticles are molded or compressed into the desired shape by coldcompression using an isostatic press at about 40 to about 80 ksi (about275 to about 550 MPa), followed by forging or sintering and machining,to provide an article having the desired shape and dimensions. Asdisclosed herein, forging or sintering is carried out at a temperaturebelow that of the melting point of the components.

Thus, a method of forming an article comprises molding the compositeparticles and forging the molded article.

The article so prepared is referred to as a controlled electrolyticmaterial (CEM) article, and useful under downhole conditions. Articlesinclude, for example a ball, a ball seat, a fracture plug, or other suchdownhole article. However, it should be understood that though thesearticles are disclosed, there are other uses for the composite particlesin powder form. For example, the composite particles may be included ina matrix that is non-metallic, and may be applied to a surface as acoating, such as a paint, powder coating, etc., where a controlledelectrolytic process occurs in the presence of water, and preferably,water plus an electrolyte. Such processes may include coatings formarine applications such as drill rigs, boat or ship hulls, underseatools, or other such applications. Such an electrolytic material mayprovide a sacrificial layer to mitigate or prevent corrosion of anunderlying metal layer, or may alternatively prevent adhesion of, forexample, marine organisms to the underwater surface coated with thecomposite particles.

An exemplary use is described herein. FIG. 1 shows in schematiccross-section different structural variants of the composite particles100 a and 100 b. In FIG. 1A, the composite particle 100 a includes acore 110; a shielding layer 120 which includes an intermetallic region121 (heavy dashed line) and aluminum layer 122 surrounding theintermetallic region; a metallic layer 130, and optionally, a secondaluminum layer 140 which functions as an adhesion layer. This adhesionlayer 140 may be included to promote the adhesion of particles whencompressed together to form a shaped article. It will be appreciatedthat the layers, while shown as discrete core-shell layers, can also beintermixed at the interfaces and/or the layers can be discontinuous onthe surfaces to which they are applied, such that core 110 is actuallyin contact with shielding layer 120 and/or metallic layer 130 and/oradhesion layer 140.

In FIG. 1B, composite particle 100 b has a core 110 and aninter-dispersed layer 150 which includes the components of theinterlayer region, shielding layer, metallic layer, and optionaladhesion layer (not shown individually in FIG. 1B). It will beappreciated that such an inter-dispersed structure can derive from adiscontinuous core-shell structure as described in FIG. 1A, and in whichinter-dispersion is enhanced by thermal treatment (e.g., sintering); orthe inter-dispersed structure can derive from a physical method offorming the particles (e.g., cryo- or ball-milling) or by includingprecursor materials for more than one layer into a fluidized bed reactorduring layer formation. The inter-dispersed layer 150 is homogeneouslyinter-dispersed with the components of the different layers (interlayerregion, shielding layer, metallic layer) equally distributed throughoutinter-dispersed layer 150, or is non-uniformly distributed, for example,in a gradient where the composition changes from predominantlyinterlayer region composition at the interface of inter-dispersed layer150 and core 110, to predominantly adhesion layer composition at theouter surface of inter-dispersed layer 150.

FIG. 2 shows, similar to FIG. 1, cross-sectional views of differentstructural variants of the composite particles 200 a and 200 b. In FIG.2A, the composite particle 200 a includes a core 210 comprising innercore 211 and outer core 212; a shielding layer 220 which includes anintermetallic region 221 (heavy dashed line) and aluminum layer 222surrounding the intermetallic region; a metallic layer 230, andoptionally, a second aluminum layer 240 which functions as an adhesionlayer. As in FIG. 1A, it will be appreciated that the layers, whileshown as discrete core-shell layers, can also be intermixed at theinterfaces and/or the layers can be discontinuous on the surfaces towhich they are applied.

In FIG. 2B, composite particle 200 b has a core 210 comprising innercore 211 and outer core 212 and an inter-dispersed layer 250 whichincludes the components of the interlayer region, shielding layer,metallic layer, and optional adhesion layer (not shown individually inFIG. 2B). As in FIG. 1B, it will be appreciated that such aninter-dispersed structure can derive from a discontinuous core-shellstructure, from milling to form the particles, or by including precursormaterials for more than one layer into a fluidized bed reactor duringlayer formation. Also as in FIG. 1B, the composition of inter-dispersedlayer 250 is homogeneously distributed, or is non-uniformly distributed,such as for example, in a gradient.

In FIG. 3, as an exemplary article, a ball 300 is shown. In FIG. 3, theball 300 is composed of composite particles 310. During cold compactingto form ball 300, the powdered composite particles 310 are compressedinto and shaped to form the spherical ball 300 with interstitial spaces320, where the interstitial spaces 320 are further reduced in volume byforging and/or sintering to reduce free volume from about 20% aftercompacting to less than about 5%, specifically less than about 3%, andstill more specifically less than about 1% after forging/sintering. Whenused in conjunction with a ball seat (not shown) and seated in the ballseat to prevent fluid flow past the ball/ball seat, ball 300 forms adownhole seal for isolating, for example, a fracture zone located belowthe ball/ball seat assembly.

While one or more embodiments have been shown and described,modifications and substitutions may be made thereto without departingfrom the spirit and scope of the invention. Accordingly, it is to beunderstood that the present invention has been described by way ofillustration and not limitation.

All ranges disclosed herein are inclusive of the endpoints, and theendpoints are independently combinable with each other. The suffix “(s)”as used herein is intended to include both the singular and the pluralof the term that it modifies, thereby including at least one of thatterm (e.g., the colorant(s) includes at least one colorant). “Optional”or “optionally” means that the subsequently described event orcircumstance can or cannot occur, and that the description includesinstances where the event occurs and instances where it does not. Asused herein, “combination” is inclusive of blends, mixtures, alloys,reaction products, and the like. All references are incorporated hereinby reference.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. Further, it should further be noted that the terms “first,”“second,” and the like herein do not denote any order, quantity, orimportance, but rather are used to distinguish one element from another.The modifier “about” used in connection with a quantity is inclusive ofthe stated value and has the meaning dictated by the context (e.g., itincludes the degree of error associated with measurement of theparticular quantity).

What is claimed is:
 1. A method for adjusting corrosion rate in anaqueous electrolyte of a composite particle having: a core; a shieldinglayer deposited on the core, and further comprising an interlayer regionformed at an interface of the shielding layer and the core, theinterlayer region having a reactivity less than that of the core, andthe shielding layer having a reactivity less than that of the interlayerregion; a metallic layer not identical to the shielding layer anddeposited on the shielding layer, the metallic layer having a reactivityless than that of the core; and optionally, an adhesion metal layerdeposited on the metallic layer, the method comprising: selecting themetallic layer such that the lower the activity of the metallic layer isrelative to the shielding layer, the greater the corrosion rate, andselecting the amount, thickness, or both amounts and thicknesses of theshielding layer and metallic layer such that the less the amount,thickness, or both amount and thickness of the shielding layer relativeto those of the metallic layer, the greater the corrosion rate.
 2. Themethod of claim 1, wherein the interlayer region, shielding layer,metallic layer, and optional adhesion metal layer are inter-dispersedwith each other.
 3. The method of claim 1, wherein the core comprisesmagnesium; the shielding layer comprises aluminum, and inclusions ofalumina, magnesia, or a combination comprising at least one of theforegoing oxides; and the interlayer region comprises an intermetalliccompound.
 4. The method of claim 3, wherein the intermetallic compoundis γ-Mg₁₇Al₁₂.
 5. The method of claim 1, wherein the metallic layercomprises a group 6-11 transition metal.
 6. The method of claim 5,wherein the group 6-11 transition metal comprises Ni, Fe, Cu, Co, W,alloys thereof, or a combination comprising at least one of theforegoing.
 7. The method of claim 1, wherein the core comprises an innercore of a first core material and an outer core of a second corematerial, the inner core material having a lower activity than that ofthe outer core.
 8. The method of claim 1, wherein the inner corecomprises aluminum, and the outer core comprises magnesium.
 9. Themethod of claim 1, wherein the core comprises a magnesium-aluminumalloy.
 10. The method of claim 1, wherein the core and shielding layer,shielding layer and metallic layer, and metallic layer and optionaladhesion metal layer are each in mutual partial contact.
 11. The methodof claim 1, wherein the shielding layer is cathodic relative to the coreand anodic relative to the metallic layer.
 12. A method for adjustingcorrosion rate in an aqueous electrolyte of a composite particle having:a magnesium-aluminum alloy core; a shielding layer comprising analuminum-containing layer deposited on the core, further comprising aninterlayer region comprising γ-Mg₁₇Al₁₂ formed at the interface betweenthe magnesium alloy core and the aluminum-containing layer, and furthercomprising inclusions of alumina, magnesia, or a combination comprisingat least one of these oxides; a metallic layer deposited on theshielding layer, the metallic layer comprising Ni, Fe, Cu, Co, W, alloysthereof, or a combination comprising at least one of the foregoing, analuminum-containing shielding layer deposited on the metallic layer; andoptionally, an aluminum-containing adhesion metal layer, the methodcomprising: selecting the metallic layer such that the lower theactivity of the metallic layer is relative to the shielding layer, thegreater the corrosion rate, and selecting the amount, thickness, or bothamounts and thicknesses of the shielding layer and metallic layer suchthat the less the amount, thickness, or both amount and thickness of theshielding layer relative to those of the metallic layer, the greater thecorrosion rate.
 13. The method of claim 12, wherein the interlayerregion, shielding layer, metallic layer, and optional adhesion metallayer are inter-dispersed with each other.
 14. The method of claim 12,further comprising molding a plurality of the composite particles. 15.The method of claim 14, further comprising forging the molded compositeparticles to form an article.
 16. The method of claim 15, wherein thearticle is a ball, ball seat, or fracture plug.
 17. The method of claim12, further comprising dispersing a plurality of the composite particlesin a matrix to form a composition.
 18. The method of claim 17, whereinthe matrix is non-metallic.
 19. The method of claim 17, furthercomprising applying the composition to a surface to form a coating. 20.The method of claim 19, further comprising performing a controlledelectrolytic process on the coating in the presence of water and anelectrolyte.